Natalie Grigorieva: The Mel Gibson Tape Source?

The now-infamous Mel Gibson tapes are as shocking as anything we've heard ... well, ever. But the mystery surrounding them has become a separate and equally bizarre component in the actor's custody fight with Oksana Grigorieva.

There may finally be an answer to how Gibson's insane rants against Grigorieva were leaked online, something Oksana has denied being responsible for.

Several sources close to the case say Oksana's younger sister, Natalie Grigorieva, is suspected of being Radar Online's mystery Mel Gibson tape source.

Natalie is being investigated by authorities.

"I suspect if the authorities look at the sister, they'll find a wire transfer or a check to her from [Radar]," a source said. "She and Oksana are very close."

Another source claims that Oksana's sister was even involved in shopping the tapes to media outlets before making the sale to the celebrity gossip site.

Did Oksana's sister leak the tapes?

"All the tabloids said no to the deal because they were asked to purchase the [Mel Gibson tapes] before even listening to them," another source said.

If Natalie Grigorieva is found to have brokered a deal at the behest of her sister, it's possible a judge could find the Russian singer in contempt of court.

Oksana Grigorieva, under investigation herself for extortion, also could face charges based on text messages and e-mails she sent to the actor.

In a text message obtained by The Hollywood Reporter, Oksana allegedly wrote Mel that she taped him because "you broke your agreement with me."

The two parties, who share an eight-month-old daughter, had agreed on a $15 million deal over custody and keeping all "evidence" confidential.

But Oksana left it on the table because, she said, it gave Gibson unsupervised visits with their child, and she feared for Lucia's safety around Mel.